1. Field of the Invention
The present invention relates to a memory array circuit for a nonvolatile memory device with two-bit memory cells.
2. Description of the Related Art
Related art is disclosed in U.S. Pat. No. 6,147,912 to Kitazawa, U.S. Pat. No. 6,233,168 to Kokubun et al., and U.S. Patent Application Publication No. 2004/0222452 by Matsuoka et al. (and in the corresponding Japanese Patent Application Publications No. 11-203880, 200-57794, and 2004-335797).
A memory array circuit disclosed by Kitazawa is illustrated in FIG. 1. The memory array comprises a plurality of subblocks and a multiplexer; one subblock (SUBBLK) 1 and the corresponding part of the multiplexer (MPX) 2 are shown in FIG. 1. The subblock 1 comprises a plurality of parallel word lines WL0, WL1, . . . , a plurality of select lines SL0, SL1, . . . that cross the word lines, and a plurality of sub-bit lines SBL0, SBL1, . . . that also cross the word lines.
Memory cells MC0, MC1, . . . are located at the intersections of the word lines and select lines (only the memory cells on word line WL0 are shown). Each memory cell has a floating gate that stores data as the presence or absence of charge, a control electrode connected to the adjacent word line, a drain connected to the adjacent select line, and a source connected to the adjacent sub-bit line.
The select lines SL0, SL1, . . . are connected through respective switching transistors referred to as drain selectors DS0, DS1, . . . to a common power supply line CDV. The gates of the even-numbered drain selectors DS0, DS2, . . . are all connected to a drain select line DSE; the gates of the odd-numbered drain selectors DS1, DS3, . . . are all connected to another drain select line DSO. The sub-bit lines SBL0, SBL1, . . . are connected through respective switching transistors referred to as source selectors SS0, SS1, . . . to respective main bit lines MBL0, MBL1, . . . .
Although not shown, a plurality of subblocks similar to subblock 1 are connected in parallel to the main bit lines MBL0, MBL1 . . . . The main bit lines MBL0, MBL1, . . . are connected through the multiplexer 2 to data lines DL0, DL1. The multiplexer 2 selects a mutually adjacent pair of main bit lines according to select signals Y0, Y1, . . . and connects them to the data lines DL0, DL1.
Sense amplifiers SA0, SA1 are connected to the data lines DL0, DL1. Although not shown, a data write circuit and other circuits are also connected to the data lines. The sense amplifiers SA0, SA1 detect the current flowing through a selected pair of memory cells to the ground level, thereby reading out the information stored in the memory cells.
FIG. 2 illustrates the readout operation when memory cells MC6 and MC9 in FIG. 1 are selected. The thick lines indicate the active signal lines and the paths of the currents flowing through the selected memory cells MC6, MC9.
As shown in FIG. 2, when the source select line SS and select signal Y3 are driven to the high logic level, two current paths are activated: one from sub-bit line SBL3 to sense amplifier SA0 via main bit line MBL3 and data line DL0; the other from sub-bit line SBL4 to sense amplifier SA1 via main bit line MBL4 and data line DL1. If word line WL0 and drain select line DS0 are also driven to the high logic level, another two current paths are activated, one from the common power supply line CDV to sub-bit line SBL3 via drain selector DS3 and memory cell MC6 and another to sub-bit line SBL4 via drain selector DS5 and memory cell MC9. Therefore, when memory cell MC6 stores the data ‘1’, for example, read current flows through main bit line MBL3 to sense amplifier SA0. When memory cell MC9 stores the data ‘1’, read current flows through main bit line MBL4 to sense amplifier SA1.
It can appreciated from FIG. 2 that when memory cells MC6 and MC9 are read, the sub-bit lines SBL3, SBL4 on the read paths may be interconnected through the on-resistances of memory cells MC7 and MC8. The voltage levels of the sub-bit lines SBL3, SBL4 are equalized by the sense amplifiers SA0, SA1 regardless of the data stored in the memory cells MC6, MC9. When the data stored in memory cells MC6 and MC9 differ, however, a voltage difference occurs between sub-bit lines SBL3 and SBL4, so leakage current flows through memory cells MC7 and MC8. Accordingly, when this memory array circuit is used, the leakage current needs to be small enough to be negligible.
The intermediate memory cells MC7, MC8 and the sub-bit lines SBL3, SBL4 used for reading data are both disposed between the memory cells MC6, MC9 being read. Therefore, aside from the unavoidable parasitic capacitance components on the read paths, such as the parasitic capacitance of sub-bit lines SBL3, SBL4 and main bit lines MBL3, MBL4, the parasitic capacitance that slows the read operation is limited mainly to select line SL4 and the memory cells MC7, MC8 connected thereto. This substantial limitation of parasitic capacitance to the circuit elements disposed between the selected memory cells dramatically reduces the total parasitic capacitance, enabling high-speed operation.
The above memory array circuit, however, is designed for use with nonvolatile memory cells storing data as the presence or absence of charge stored in a single floating gate, and having a fixed source electrode and drain electrode.
With the recent increase in demand for large memory capacity, nonvolatile memory elements that can store two bits of information in a single memory cell have emerged.
A two-bit memory cell described by Matsuoka et al. is illustrated in FIGS. 3 to 7.
As shown in the sectional view of FIG. 3, the memory cell has a gate electrode 13 insulated by a gate oxide layer 12 from the surface of a p-well region 11, and charge storage regions 14L, 14R comprising silicon nitride films formed on the sidewalls of the gate electrode 13. N-type diffusions 15L, 15R formed at the surface of the p-well region 11 extend partly below the charge storage regions 14L, 14R. N-type diffusion 15L can be used as a source electrode and 15R as a drain electrode or, reversely, n-type diffusion 15R can be used as a source electrode and 15L as a drain electrode, depending on the applied voltages.
FIG. 4 illustrates an operation that writes data into the left n-type diffusion 15L of the memory cell. The right n-type diffusion 15R is used as the source electrode and the left n-type diffusion 15L as the drain electrode. For example, the right n-type diffusion 15R and p-well region 11 are biased to zero volts (0 V), and the left n-type diffusion 15L and gate electrode 13 are biased to +5 V. An inversion layer 16 extends from the right n-type diffusion 15R, but is pinched off before reaching the left n-type diffusion 15L. Electrons are accelerated by a high electric field from the pinch-off point to the n-type diffusion 15L, becoming so-called hot electrons. The hot electrons are injected into the left charge storage region 14L and stored as data. No hot electrons are generated in the vicinity of the right charge storage region 14R, so no write operation is performed there.
To write data into the right charge storage region 14R, the left n-type diffusion 15L is used as the source electrode and the right n-type diffusion 15R as the drain electrode.
FIG. 5 illustrates a read operation. To read the information stored in the left charge storage region 14L, the memory cell transistor is operated using the left n-type diffusion 15L as the source electrode and the right n-type diffusion 15R as the drain electrode. For example, the left n-type diffusion 15L and p-well region 11 are biased to 0 V, the right n-type diffusion 15R is biased to +1.8 V, and the gate electrode 13 is biased to +2 V. If no electrons are stored in the left charge storage region 14L, an inversion layer 16 forms and drain current flows readily. In contrast, if electrons are stored in the left charge storage region 14L, hardly any inversion layer forms therebelow, so hardly any drain current flows. Accordingly, the information stored in the charge storage region 14L can be read out by detecting the drain current. To read the information stored in the right charge storage region 14R, the memory cell transistor is operated using the right n-type diffusion 15R as the source electrode and the left n-type diffusion 15L as the drain electrode.
FIG. 6 illustrates an erase operation. To erase the information stored in the left charge storage region 14L, the pn-junction between the left n-type diffusion 15L and p-well region 11 is reversely biased by positively biasing the n-type diffusion 15L (for example, to +5 V) with respect to the p-well region 11, which is biased at 0 V. In addition, the gate electrode 13 is negatively biased (for example, to −5 V) and the right n-type diffusion 15R is biased to 0 V. The potential gradient at the pn-junction thereby becomes especially steep in the vicinity of the gate electrode 13 due to the effect of the negatively biased gate electrode, which induces band-to-band tunneling and generates hot holes on the p-well side of the pn-junction. The hot holes are attracted towards the negatively biased gate electrode 13 and injected into the left charge storage region 14L, erasing the information stored therein. To erase information stored in the right charge storage region 14R, it suffices to interchange the potentials of the charge storage regions 15L, 15R.
As described above, in a two-bit memory cell, charge storage regions 14L, 14R are formed on the left and right sidewalls of the gate electrode 13, and corresponding left and right n-type diffusions 15L, 15R are used as source and drain electrodes, respectively, or as drain and source electrodes, respectively, to store two bits of information.
This type of memory cell cannot be used in the conventional memory array circuit illustrated in FIG. 7, because the select lines must always be connected to the drain electrodes and the sub-bit lines must always be connected to the source electrodes of the memory cells.